Mic-Encoder use with a voice repeater. The mic-encoder is a device that sends a 1/3second APRS data burst at the end of a voice transmission. It is like the "roger beep" some CB'ers use.
Mobile MIC Encoder Hardware
TAPR has discontinued the original mic encoder, however the tiny track and opentracker devices are in production and may be wired and configured to send position data at the end of a voice transmission. These devices can be programmed to send positions at the end of a voice transmission but not necessarily every voice transmission.
For example, a mic encoder device can be programmed to send a position report every 3 minutes - but only at the end of a voice transmission. When the radio operator keys up to talk for the first time, a data burst will be sent at the moment the PTT is released. He can key the radio up several more times and no additional burst will be sent until more than 3 minutes have passed since the last data burst. The mic encoder unit will not key up on it's own at the end of 3 minutes and force a position. This completely eliminates the posibility that a packet burst will blindly "double" with another active voice user. If the encoder is programmed to send a position at a shorter interval, such as every 10 seconds it will transmit a databurst every time the radio unkeys. The mic-e will never cause a "double" with another repeater user, nor will it send a data burst over the top of the radio operator's voice while he is talking.
Methods to convey MIC Encoder data bursts to the APRS frequency
This data burst can be read by any station listening to you direct or via the output of a repeater you happen to be talking through.
The simplest way to these position reports from the repeater to the normal aprs frequency of 144.39 is to place a police scanner within range of the repeater connected to a TNC. The TNC is configured to recieve audio from the police scanner or other receiver and transmit to a 2m transmitter. The TNC upon hearing the packet on the police scanner (or other receiver) will digipeat the packet out on the normal APRS frequency. It is recommended that the deviation level of mic-e packets be kept very low so the unmuted packet tones don't "blast" anyone listening to the repeater.
Even though the data bursts are only 350mS long, after some time data bursts on the output of a repeater will begin to annoy some voice users. If the repeater is properly equipped, the data can be muted, and repeated to the standard APRS frequency (usually 144.39 Mhz) unbeknownest to the normal voice repeater users. If implemented on a nationwide level, this would allow all APRS users in the country to see all mobile aprs equipped cars.
The ideal TNC to use at the repeater site is presently the Kantronics kpc3plus. By issuing the command "CTRL A MUTE 10" you can cause radio port pin 8 to become a mute pin to mute audio on your voice repeater. A kpc3(nonplus) model can also be made to perform the same function with a relatively minor hardware modification. The newest KPC3+ model has version 9.1 firmware, but the MUTE function has been available since version 8.3. Either firmware supports analog inputs, digital outputs, and advanced APRS digipeater functions. This makes it possible to send telemetry from the repeater site in addition to APRS position data.
PACCOMM brand TNCs have had this ability for some time now as long as they have the true DCD board installed. This has been known to false though and could mute someone's voice at the wrong time - just as some people's voices cause the DTMF cover tones to activate on a voice repeater.
The DCD or MUTE signal can be tied to the repeater receiver audio or the COR signal. The COR signal is the preferred method because many repeater controllers have audio delay processors and can mute the audio before even the first millisecond of packet data is passed to the repeater output frequency. If the repeater is not equipped with an audio delay line, the 10mS of packet will sound similar to the "blip" heard before a DTMF cover tone sounds.
There are two common methods to interrupt the COR signal:
- with a reed relay.
- by installing a 100ohm resistor in series between the receiver and repeater controller and allowing the TNC to clamp the repeater controller end of the signal to ground or Vcc.
Since many repeater controllers such as the CAT1000 use a DB25 plug for the interface between the radio and the repeater controller, a rs232 db25 breakout box with a db25m at one end and a db25f at the other. Radio shack used to sell these for about $10, but have discontinued them. One alternative is http://www.cablesandconnectors.com/PIX/phi23 near the bottom of the page.
Is is necessary to install a 2m radio at the repeater site with a small 5/8 or 1/4 wave antenna on the roof of the repeater shack. Care should also be taken to provide a squelch indication from the 2m radio to the external carrier detect (XCD) pin on the TNC. This will allow the TNC to hold off digipeating the mic-e packet into the APRS frequency until that channel is clear.
In addition to the first example listed above, the mic encoder units can be programmed to wait for quiet time on the repeater and automatically send a position every 10 to 30 minutes even if the user doesn't manually key up his radio. If the repeater controller's COR signal is used to mute audio, users can sent positions via the repeater all day long without activating the repeater output!
Consider that the average repeater has a "kerchunk" filter that requires the COR is active for 0.5 sec before the repeater will wake up from a dormant state. If a mic-encoder device keys up and transmits a packet to a repeater with COR muting, within 10mS of packet starting, the TNC at the repeater site will cause the COR line to drop. This happens within 10mS of the beginning of the packet. The repeater controller will see COR go active for 10mS, and drop. The repeater controller thinks it was kerchunked, and does not activate the repeater output. Since the repeater output was not activated, no one listening to the output of the repeater is aware that any data was sent on the input frequency. In effect, the packet is silently diverted over to the normal APRS frequency.
It gets better! Many TNCs today perform callsign substitution when digipeating. If the callsign of the TNC at the repeater site is set to the frequency of the repeater - up to 6 characters - then any packet passing though the repeater will be marked with the voice repeater's frequency. For example, by setting the callsign of the TNC to 147015, any packet digipeated via the input of the repeater has the text 147015 added to it's path and it is clear to APRS users that:
- this position was from a mic-e packet,
- the user is monitoring the 147.015 repeater. If the user QSY's to another mic-e capable repeater, the TNC at that site will add it's own stamp to the APRS packet. Because the frequency stamp information is added at the repeater level, the voice user does not have to make any changes to his mic encoder device. Other APRS users are instantly aware of the station in question's frequency.